I have some questions about how armor components work. I only have the core rules and not the armor book, so perhaps it's clarified there. In any case, the components table lists "Stand-Alone DR" but it's not explained anywhere what exactly that means. Also, several footnotes on the table seem to be referencing things that aren't there, like DR/HP bonuses. I take it that adding components to Partial/Half/Medium armor is supposed to add something to your total armor DR and HP but what? It seems unlikely that it's the Stand-Alone DR number because they are so high.

Stand-alone DR is there for if that is all you are wearing. I.e. if you wear an iron helmet and nothing else, the DR is 8, though your bypass is only 1 (so it's easy to get around it).

Technically, according to the core rules, armor components can only add to the bypass value of your armor, not the DR. So if you are already wearing a heavy gambeson (4/8/4 bypass 4) and add a pair of iron bracers, your bypass* goes up to 5. There would be no other effect. That is the simple version.

You could however also get a little deeper into it if you want to. If you for example take a guy with a heavy gambeson, and add an iron helmet, then you can do some math. The iron helmet amounts to 1/5 of the overall coverage (bypass 1 out of a total of 5) but it's DR is 8, so if you took the DR of 8 and added it to 16 (four times the default DR of 4 for the gambeson) = 24, and then divide by 5, you end up with an 'average' DR of 4.8, which you could call DR 5. This way adding the helmet would increase the DR a little in addition to boosting coverage (bypass penalty).

So there is either a 'quick and dirty' option for adding miscellaneous armor components by just adding some coverage, or you can get deeper into it and compute a new 'average' DR value as well based on all the pieces off armor and how much they cover, per the above.

The armor book adds another optional variant to the armor rule, to basically allow two tiers of armor protection at different DRs which is a little more realistic. So in that version, your helmet would still have DR 8, but the bypass would only be 1 for the helmet, while your gambeson (only DR 4) has a bypass of 4. An attacker could decide whether to avoid just your helmet, or all your armor. Since most forms of armor have stronger and weaker areas, this gives you a more nuanced option for bypass (and for setting up your protection).